1. Field of the Invention
The present invention relates to devices for measuring airflow rates. More particularly, the present invention relates to devices for measuring airflow rates using a capacitive device. Still more particularly, the present invention is a relatively inexpensive system designed for reliable accuracy at very low flow rates.
2. Description of the Prior Art
The flow rates of gases are of significant importance in a wide variety of medical and industrial applications. For the purpose of the description of the present invention, "gas" means any non-liquid fluid, including, but not limited to air. It is often necessary to measure gas flow rate to ensure that an expected volume of gas desired to be specifically transferred from one location to another is the volume required for an intended purpose. Alternatively, it may be necessary to undertake such measurement to determine the amount of what would otherwise be an unknown volume of gas so as to evaluate the effect of that transferred gas in a particular application.
One area among many others in which gas--air in particular--flow rates are of importance is the medical field. Another area where gas flow rates are of importance is in laboratory fume hoods. Laboratory fume hoods are used to protect laboratory personnel from exposure to toxic and pathogenic substances. Proper operation of the fume hood relies on a specific air flow rate at the opening of the fume hood and at other locations within the hood. If the air flow rate is too low, the hood does not provide adequate isolation of the substances contained therein. If the flow rate is too high, the substances may be contaminated or otherwise compromised by the entry of ambient laboratory air.
As often as not, these hoods or cabinets include fans that blow from one confined area through a working area section and into an exhaust section. Holes created between the respective sections can be used to evaluate gas flow rates as indicators of the pressure differential between those sections. It is therefore critical to measure gas flow rates in these cabinets in order to determine whether the pressure differential meets a particular design point. If the rate is too low, the hood is not safe for laboratory personnel. If it is too high, the experiment being conducted in the working section of the cabinet may be compromised. It is therefore important to set design gas flow rates and to ensure that they are maintained. As a result of these requirements, gas flow measurement devices have been developed, including those designed to measure airflow rates.
In addition to the use of anemometers, rotary, and sonic systems, among others, a number of the prior gas flow measurements devices have been based on the use of a pressure sensor. A pressure sensor is configured to respond to some pressure differential created by the flow of a gas. Low gas flows create small pressure differentials. Low-pressure range transducers are more difficult and expensive to fabricate and are subject to thermally-induced errors. As much as they are designed to compensate to minimize the effects of ambient temperatures, practical and commercially-available pressure transducers exhibit a significant temperature coefficient. That is, the results obtained from such transducers vary widely as a function of temperature variations. This limits their usefulness in measuring low flow rates, where variations are more significant.
Another type of gas flow measurement device is the capacitive sensor. This type of sensor also includes an electrically-conductive component designed to move with the movement of gas passing by. However, the moving component is made part of an electrical circuit such that when the position of the component changes, an insulative gap between it and another electrically-conductive component of fixed position also changes. The resultant change in capacitance is used to determine a gas flow rate. Several such devices have been previously described. U.S. Pat. No. 4,599,907 issued to Kraus et al. describes a mass-flow sensor including a flexible plate positioned in the field of the gas flow, and a fixed plate. The combination of plates produces a variable capacitor. The mass flow rate of the gas is proportional to the rate of change of the measured capacitance. U.S. Pat. No. 5,003,810 issued to Jepson et al. describes a mass flow meter for gases. The device uses a plate located in a flow stream and coupled to circuitry for evaluation of capacitive changes. European Patent Application No. 86309946.1 describes a capacitive device including a semiconductor element and a flexible beam, the movement of which results in a change in capacitance.
An important limitation of these and other prior-art devices is the accuracy of measurement for very low flow rates, namely, for flow rates on the order of 300 feet per minute or less. When the flow rate to be maintained or otherwise determined is that low, the designs of the prior devices restrict their usefulness. Among other things, the movable components of such devices may be too stiff at such rates, in order not to be too flexible at higher flow rates. In addition, at low flow rates, the movable component will have an effect on the flow rate and therefore render its measurement relatively inaccurate. Further, the devices presently available for very low flow rate measurements tend to be relatively expensive in that they often rely upon sophisticated circuitry and software to account for the deficiencies in their structural components. Such device sophistication can lead to high maintenance requirements and to the need for sophisticated operators.
Therefore, what is needed is a device for accurate measurement of gas flow rates at relatively low flow rates. What is also needed is such a device that is relatively temperature insensitive. Further, what is needed is such a device that is relatively easy to fabricate and use in a cost-effective way.